for one they are worth more money, 2nd they are harder to get, which makes them worth more money, also with the whole harder to get thing, traders will trade well for cards they know are hard to get and greatly wanted by other traders.

The reason for Foils is that less are produced (supposedly) than the regular non-foil versions of the same cards.

Then players like to use these foils in their decks. I don't know why exactly.
So you have the basic economics of:
less foil versions made & in circulation (low supply)
+
increased demand from players And Collectors
=
higher priced foil cards (compared to non-foils)

I think the game companies should just make ALL cards foil, but then what carrot would they have to hold before our noses?